One of the things people who don't like soccer always say about why they don't like it is that it's low scoring. Another is that it's boring. Whenever I am talking with someone who has this set of objections to soccer, I always point them to one game - the 1999 FA Cup semifinal.
Not the one you're thinking of, with the Schmeichel save of a Bergkamp penalty that would have won the tie and the Giggs goal. The one I'm thinking of is the 0-0 that led to that replay. I have to this day yet to see a better scoreless draw - it was played at about a million miles an hour, with a ton of chances for both sides, and was a pretty good advertisement for soccer to anybody who needed to be convinced it could be a fun game to watch.
So, it's safe to say these two sides have some history in the Cup. That history doesn't really come into play tomorrow except in the media, of course; what does come into play tomorrow is that Arsenal have yet again put themselves in a position where the FA is the most likely of Cups they can win this season.
Manchester United will be without their wounded little boy, and Luke Shaw is questionable with an ouchy hamstring; Jonny Evans starts the most ludicrous ban ever, sitting the first of his six games he got for spitting at Papiss Cisse (while Maynor Figueroa got exactly no punishment for eviscerating Steven Ireland's leg the same weekend). Szcz and Nacho face late fitness tests, but otherwise Arsenal are as full-strength as they were last weekend.
My love for the FA Cup is well known, and matches like these are only one facet of why I love it so much. I want Arsenal to win it, of course, but after watching that semi in 1999, I also want Manchester United to lose it; if both of those can be done by Arsenal, so much the better.